Cable Sizing considerations for Exciter Cables

[rockman7892]

I am currently reviewing a project in which the cables between the generator Exciter Compartment (AVR) and the generator field compartment are being replaced. I was curious if there were any guidelines or references for verifying or confirming the required ampacity of these cables?

Generator is a 230kVA Gas Turbine unit with its Field Amps listed as 1,870A @ 375V DC. The exciter itself has a listed continuous ampacity of 2150A @ 420V DC and a Ceiling of 3400A @ 576 VDC.

Does this exciter cable typically have its ampacity sized to meet the rated field amps based on the generator nameplate, or is it sized based on the capability of the exciter itself? I would think its based on the rated machine field amps but wanted to make sure I wasn't missing something.

If determining the ampacity based off the rated field amps is it typical to use the field amps at generator full load (1,870A) or do you typically put some margin on this value such as 105%, 110%, etc...? Generator datasheet does not provide specific overload values (105%, etc...) but does provide saturation curves with Field Amps vs Armature Voltage/Current. There are multiple curves on this plot with two of the curves showing the field amps above the rated field amps for conditions labeled as "Saturation at rated armature amps-rated PF" and "Saturation at rated arm amps - 0 PF". Doe these saturation considerations need to be taken into account when determining required ampacity of cable?

Thanks for the help!

 

[melspuds]

I would think that cable size goes by rated field amps of the generator. The next question is, what is the distance from the exciter to the generator in cable length from connector to connector? With length and ambient temperature you can calculate the cable size and I would go at least 10 to 15%+ for final cable size.
From past experience with large DC loads, the voltage drop in the cables is a killer.
Is bus bars an option?
Hope this helps,
Dave

 

[ScottyUK]

Cable size would be determined by the setting of whatever protects the cables. The rotor will probably have a longer time constant than the cable, so the limiters designed to protect the rotor may not protect the cable. Your client will want to maximise the capability of the generator, so sizing the cable to allow operation in the extreme areas of the capability diagram is a realistic requirement. You also don't want to limit the AVR's ability to control the field under difficult conditions. Does the AVR have any field forcing capability where the AVR can exceed steady state output by a considerable margin for short duration? Volt-drop may become a dominant factor udner field forcing conditions.

I agree that bus duct is a good option, especially at a relatively low voltage and relatively high current. Cabling is going to be awkward with parallel runs of large conductors.